In the manufacturing of wafers, integrated circuit devices such as transistors are first formed at the surfaces of semiconductor substrates. Interconnect structures are then formed over the integrated circuit devices. Bumps are formed on the surfaces of the semiconductor wafers, and are electrically coupled to integrated circuit devices. The semiconductor wafers are sawed into semiconductor chips, also commonly known as dies.
In the packaging of the semiconductor chips, the semiconductor chips are often bonded with other chips or package substrates using flip-chip bonding. Solders are used to join the bumps in the semiconductor chips, or join the bumps in the semiconductor chips to the bond pads in the package substrates. When two semiconductor chips (or one semiconductor chip and a package substrate) are bonded, a solder bump may be pre-formed on one of the bumps or bond pads. A re-flow is then performed so that the solder bump joins the semiconductor chips. Conventional bumps were typically large, and hence micro-bumps were developed. Micro-bump flip-chip interconnections allow for high bonding densities.
FIG. 1 illustrates an exemplary micro-bump on chip 200, with micro-bump 210 formed at a surface of chip 200. Micro-bump 210 includes nickel layer 212, and copper pad 214 on nickel layer 214. Silicon nitride layer 216 covers the edge portion of micro-bump 210, with a center portion of the top surface of copper pad 214 not covered. Electroless nickel electroless palladium immersion gold (ENEPIG) finish 220 is formed to cover copper pad 214, which is exposed through the opening in silicon nitride layer 216. Bump 210 as shown in FIG. 1 will be joined with bump 230 of chip 232 by re-flowing solder cap 234, so that chips 200 and 232 are bonded together.